The present invention relates to a mechanical switch circuit comprising contact conditioning means which is ideally suited to a system for remote reading of utility meters or the like.
The high cost of personnel for reading utility meters has led to the development of remote reading systems. Taking a water meter as an example, a mechanical switch may be connected to the internal gearing of the meter such as to close each time one liter of water is consumed. The switch is connected through cables to a remote utility station which senses each time the switch is closed.
These systems are coming into widespread use due to their inherent advantages over direct reading. However, several problems have remained heretofore unsolved which have prevented complete acceptance of such systems.
The first problem is the sensitivity of the system to noise produced in the cables connecting the meter switches to the remote utility station. Since these cables must often be quite long, many opportunities for the introduction of electrical noise are present. Noise pulses cause erroneous operation of the sensing means in the utility station and result in overly high readings and overbilling of the utility consumer. The time required for investigation and correction of the erroneous billing may exceed the time required for direct reading of the meter, thus obviating the advantage of the remote reading system.
Another related problem is erroneous operation of the sensing means caused by bouncing of the switch contacts. When a mechanical switch is closed, the contacts bounce against each other as many as several hundred times before mechanical and electrical equilibrium is reached. Each opening and closing of the contacts gives rise to an electrical pulse which can be counted by the remote sensing means. Thus, contact bouncing also produces erroneously high readings and overbilling.
The bouncing phenomenon becomes worse with time due to the deterioration of the switch. A related but opposite phenomenon is the increase in the contact resistance caused by deterioration. In extreme cases the resistance may become so high that some switch closures may not be detectd by the sensing means. The combination of contact bouncing which produces erroneously high readings and increased contact resistance which produces erroneously low readings may result in utility readings which are totally unrelated to the amount of the utility consumed.
The effects of contact bouncing and deterioration may be minimized by passing a large amount of current through the switch contacts. This causes temporary partial fusion of the contact surfaces which maximizes the contact efficiency and rejuvenates the contacts at each closure. Although it is desirable to provide such an effect in a remote utility reading system, such has been heretofore been impractical due to the high power requirement.